Original paper
Natrophosphate from the Aris phonolites, Windhoek, Namibia
Petersen, O. V.; Fockenberg, T.; Toft, P. C.; Rattay, M.
Neues Jahrbuch für Mineralogie - Monatshefte Jg. 1997 Heft 11 (1997), p. 511 - 517
14 references
published: Nov 12, 1997
DOI: 10.1127/njmm/1997/1997/511
ArtNo. ESP156199711003, Price: 29.00 €
Abstract
Natrophosphate has been identified in the Aris phonolites between Windhoek and Rehoboth, Namibia. At this, the third occurrence of this rare species, the natrophosphate forms a glassy vein filling in a single 40 cm large boulder found in the Aris East quarry in September 1995. It forms aggregates that were colourless and transparent when found. Mainly due to dehydration, they are now milky white to greyish white and translucent and are covered by a layer of powdery white material consisting of villiaumite and secondary hydrophosphates. Associated minerals include makatite, villiaumite and tuperssuatsiaite. The chemical analysis of the natrophosphate from Aris corresponds well with those previously published for natrophosphate from Khibiny and Lovozero alkalic massifs, Kola peninsula, Russia, and with that of the corresponding synthetic compound. The empirical formula calculated by normalizing the sum (Na + P + S) to 9.00 is: Na7.04((PO4)1.87(SO4) 0.09)Σ1.96 (F1.28Cl0.39)Σ1.67 · 19.29H2O. For the first time sulphur and chlorine were detected in natrophosphate. The high amounts of fluorine plus chlorine result in a surplus of negative charges and the presence of molecules of PO3F, PO3Cl, SO3F and/or SO3Cl is tentatively offered as a way of overcoming this problem. The unit cell parameter of the natrophosphate from Aris was found to be: a = 27.93 ± 0.05 Å and corresponds reasonably with values given for both natural natrophosphate and the corresponding synthetic material. The refractive indices of the natrophosphate from Aris, n = 1.450 ± 0.005 is low but within the range given by FLEISCHER et al. (1984).
Keywords
Natrophosphate • Aris phonolites - Namibia